The newly commissioned Orion laser system has been used to study dense plasmas created by a combination of short pulse laser heating and compression by laser driven shocks. Thus the plasma density was systematically varied between 1 and 10g/cc by using aluminum samples buried in plastic foils or diamond sheets. The aluminum was heated to electron temperatures between 500eV and 700eV allowing the plasma conditions to be diagnosed by K-shell emission spectroscopy. The K-shell spectra show the

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... ct of the ionization potential depression as a function of density. The data are compared to simulated spectra which account for the change in the ionization potential by the commonly used Stewart and Pyatt prescription and an alternative due to Ecker and Kröll suggested by recent X-ray free-electron laser experiments. The experimental data are in closer agreement with simulations using the model of Stewart and Pyatt.